Light Grasp Calculator

Calculate Light Gathering Power for Reflectors and Schmidt-Cassegrain Telescopes

Telescope Specifications

Typical values: 15-25% for SCTs, 10-15% for Newtonians
Calculation Formula
Light Grasp = π × (Effective Radius)²
Effective Radius = (Aperture - Central Obstruction) ÷ 2

Where:

  • π (Pi) ≈ 3.14159
  • Aperture = Telescope primary mirror/lens diameter
  • Central Obstruction = Secondary mirror diameter as percentage of aperture
  • Effective Radius = Clear aperture radius available for light collection
Common Telescope Sizes (Inches)
Aperture Light Grasp With 20% CO
4" 12.57 in² 8.04 in²
6" 28.27 in² 18.1 in²
8" 50.27 in² 32.17 in²
10" 78.54 in² 50.27 in²
12" 113.1 in² 72.38 in²
14" 153.94 in² 98.52 in²
16" 201.06 in² 128.68 in²
Common Telescope Sizes (mm)
Aperture Light Grasp With 20% CO
100 mm 7853.98 mm² 5026.54 mm²
150 mm 17671.44 mm² 11309.72 mm²
200 mm 31415.9 mm² 20106.18 mm²
250 mm 49087.34 mm² 31415.9 mm²
300 mm 70685.77 mm² 45238.9 mm²
350 mm 96211.19 mm² 61575.16 mm²
400 mm 125663.6 mm² 80424.7 mm²
Understanding Light Grasp: Light grasp represents the telescope's ability to collect light, directly affecting how faint objects can be seen. A larger light grasp means brighter images and better visibility of dim deep-sky objects. Central obstruction reduces effective light gathering area.